1. Field of the Invention
The present invention relates to a production cell having a plurality of work performing elements, such as a robot, processing machine, or the like.
2. Description of the Related Art
In a production system for machine parts, or the like, a sequence of work tasks is carried out automatically, whereby a workpiece is received, processed, and then passed on to the next process. A production cell forming a production system that is highly automated is based on the operation of work performing elements which actually carry out the production tasks, such as a plurality of industrial robots or processing machines, or the like. The industrial robots and processing machines each have a work program, and operate individually, and they are connected mutually by communications means, such as I/O units, or the like.
When designing a production system, the production steps in each production cell are planned, the production tasks in each production step are planned, the work performing elements, such as industrial robots, processing machines, and the like, required for each production task in each step are planned, and work tasks are planned to be carried out by the individual work performing elements, such as the industrial robots, processing machines, and the like. In other words, the production work required by the production cell is achieved by dividing it up into individual production tasks for the work performing elements, such as industrial robots, processing machines, and the like, which are the constituent elements of the production cell.
A cell controller which controls the production cell indicates, via the I/O units, or other such communications means, when a work performing element, such as a robot, machine, or the like, should execute its allocated task. For example, the behaviour of a cell controller in one of a plurality of production cells performing machine processing will be considered. A robot receives a workpiece processed in an upstream cell, by means of a temporary workpiece stand, or a conveyance device. It grasps the workpiece, installs it on a processing machine, and then waits until processing has completed. When the processing by the machine has been completed, the workpiece is removed from the processing machine and is conveyed by means of a temporary workpiece stand or a conveyance device, to a downstream production cell. The cell controller controls this production step, and the cell controller performs the operations of: receiving an indication that the preparation of the workpiece from the upstream cell has been completed, by means of an input signal, notifying the processing machine to prepare for processing, by means of an output signal, receiving a processing preparation completed signal from the processing machine by means of an input signal, requesting the robot to grasp the workpiece from the upstream step and install it in the processing machine, by means of an output signal, receiving an indication that the workpiece has been installed in the processing machine from the robot, by means of an input signal, outputting a processing start instruction to the processing machine by means of an output signal, receiving a processing completed signal from the processing machine, and instructing the robot to remove the workpiece from the processing machine and supply it on to the downstream step, and when the cell controller is notified by the robot that the supply of the workpiece to the downstream step has been completed, then it waits until another workpiece is supplied from the upstream process.
In the work program of the robot, firstly, the robot waits until it receives a signal from the cell controller indicating the arrival of a workpiece from the upstream cell, and it then performs the tasks of grasping the workpiece, conveying the workpiece to the processing machine, and installing the workpiece in the processing machine, whereupon it outputs a task completed signal to the cell controller. Thereupon, the robot waits for the cell controller to send a request for the workpiece to be removed from the processing machine, upon which it performs the task of removing the workpiece, and then the task of supplying the workpiece to the downstream cell.
The foregoing operation of a production cell refers to the most simple example, involving a single processing machine and a single robot, and the production work comprises a single predetermined sequence, starting with the arrival of a workpiece from the upstream cell, and ending with the output of the workpiece to the downstream cell. In general, since the processing machine takes a long time to process the workpiece, the production capacity of the production cell is determined by the capacity of the processing machine. If the capacity of the production cell is insufficient, then the number of processing machines is raised to two, and a composition comprising a single robot and two processing machines is adopted. In this case, the program for the robot and the program for the cell controller are much more complicated than in the case of a single processing machine and single robot scenario. The cell controller monitors three states: the arrival of a workpiece from the upstream cell, the completion of processing by processing machine 1 and the completion of processing by processing machine 2. When a workpiece arrives from the upstream cell, it is investigated which of the processing machines 1 and 2 is not in use, and the work is installed in the machine that is not currently processing. In this case, it is also necessary to recognize situations where both of the processing machines are in use and the workpiece cannot be installed, and temporary stands must be provided, for example, for each of the processing machines. The robot program also becomes complicated, and must simultaneously monitor four signals: a request to install a workpiece on processing machine 1, a request to install a workpiece on processing machine 2, a processing complete signal from the processing machine 1 and a processing complete signal from processing machine 2, and it must change the contents of its work in accordance with the signal that arises.
What would be the situation if the number of processing machines in the production cell were raised to three? In this case, the processing capacity provided by the three machines may outstrip the capacity of the robot to install and remove workpieces. If this is the case, then the number of robots is increased, and the production cell will be constituted by three processing machines and two robots. When a workpiece arrives from the upstream cell, the cell controller must identify a robot that is not in use and a processing machine that is not in use, and it must then issue operating commands to this robot and processing machine. If a processing complete signal arrives from any one of the processing machines, it must identify a robot that is not in use and command it to remove the workpiece. In this way, the more complex the work operations in the production cell become, the greater the burden on the cell controller, and if the composition of the production cell is to be changed, or if the workpieces for machine components, or the like, that are to be processed by the production cell are changed, then it becomes very difficult to plan production.
Conventionally, when carrying out processing in a production cell of this kind, programs for controlling the operation of the robots and processing machines are stored respectively as sequential programs in such a manner that the work tasks are executed from the start of production until the end. Therefore, if the type of product that is to be produced is changed, or the like, then the whole program must be revised. However, in order to be able to adapt readily to a change in the type of product, or the like, Japanese Patent Application Laid-open No. 9-57576 discloses a method wherein tasks of a plurality of different types are executed sequentially and the production plan can be changed easily, by adapting a numerical control device for controlling the processing machines to each type of workpiece and each type of work task, storing a plurality of processing programs containing system constants relating to each type of workpiece and each type of work task, and supplying a processing program and the established values for the system constants to the numerical control device, from a cell controller.
Even in the case of a relatively simple production cell using processing machines and robots of this kind, the cell controller for controlling same, and the programs for the robots and processing machines forming the work performing elements, become very complicated. In this method, the cell controller and the work programs for the robots describe a sequence of production operations from start to finish, these programs being stored in storage devices of the cell controller or robots, and being read out from same and executed, and therefore it is not possible to combine them freely when executing individual work tasks on their own. In Japanese Patent Application Laid-open No. 9-57576 mentioned above, a processing program is prepared for each type of product to be processed, and by selecting a processing program, a particular sequence of production tasks is executed, but it is not possible freely to combine and execute individual task units.
In particular, if the constituent elements of the processing cell change, or if the production steps change, then it is necessary to cause the constituent elements of the system to operate according to a different procedure, and hence the program of the cell controller must be rewritten accordingly. In accordance with the changes made to the program of the cell controller, new programs must also be prepared for the robots and processing machines. Therefore, if the number of processing machines or robots is increased or decreased, or new processing steps are added, from the viewpoint of the production capacity of the cell, then it will be necessary to change the programs of all the constituent elements of the system.
Moreover, the constituent elements of the system operate on the basis of input and output communications using digital signals. Supposing that the output signal 1 from the cell controller is a request to robot 1 to convey a workpiece from an upstream cell, then this is taken to be connected to the input signal 1 of the robot 1. If the output signal 2 of the cell controller is a request to robot 2 to convey a workpiece, then this is taken to be connected to the input signal 2 of the robot 2. In this way, with regard to such digital signals, it is necessary to manage the correspondences between the meaning of the signals and the signal numbers, for each one of the respective constituent elements. Therefore, each time the number of elements is increased, or the processing step is changed, new signals are required, and on each occasion, the correspondences between signal numbers and meanings must be reviewed. Therefore, in many cases, the signal meanings and signal numbers are defined incorrectly at the stage of rewriting the programs for the cell controller, the robots and the processing machines. Furthermore, even if a workpiece conveyance processing program created for robot 1 is to be reused for robot 2, for example, since the correspondence between signal numbers and meanings is different for each robot, the program cannot be reused without modification.
Moreover, only the cell controller is able to issue work commands to the robots and processing machines, and the cell controller is not able to issue these work commands unless it follows a previously determined procedure. Therefore, it is not possible, for instance, to execute only a portion of the processing step. Furthermore, if many different types of product are to be manufactured, then previously determined procedural programs are prepared for each type of product, and it is not possible to carry out manufacture of plural types of products, in a parallel fashion.